Low dark current photoconductive device



Patented Oct. 31, 195? 3,350,595 LOW DARK CURRENT PHOTOCONDUCTIVE DEVICE William M. Kramer, Lancaster, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Nov. 15, 1965, Ser. No. 507,728 Claims. (Cl. 313-94) The invention relates to photoconductive devices and particularly to an improved target for such devices.

One type of photoconductive device in which the invention is particularly useful is the vidicon pickup tube. Vidicon pickup tubes generally comprise an evacuated glass bulb within which is mounted an electron gun of any suitable construction for developing a beam of electrons. These electrons are focused and deflected in a desired scanning pattern over a photoconductive tar-get.

The photoconductive target is usually supported on a light-transparent backing which may be the end wall or faceplate of a glass bulb or tube envelope. The target usually includes a highly conductive light-transparent layer or signal plate, which is applied to the inner surface of the faceplate facing the electron gun, and a layer of photoconductive material over the transparent conducting layer. The focused and deflected electrons are deposited on the face of the photoconductive target remote from the signal plate to produce a charge thereon.

Where panchromatic spectral response is desired, the photoconductive material may comprise a vitreous alloy including selenium, tellurium, and arsenic. This alloy possesses good spectral response and high sensitivity.

One way in which the sensitivity of a photoconductive material can be measured is by a determination of the ratio of microamperes passing through the photoconductor material, to each microwatt of radiant energy striking the material. Where this ratio is relatively large such as of the order of from .05 to .10 over a relatively large region of the spectrum including the blue and red regions thereof, the photoconductive material is usually considered to possess good spectral response and high sensitivity.

However, while a photoconductor made of the vitreous alloy of selenium, tellurium, and arsenic exhibits good spectral response and high sensitivity it is characterized by an objectionably large dark current and poor decay.

The dark current through a photoconductor constitutes leakage of electrons from the charged surface of the photoconductor to the signal plate in the dark or in the absence of radiant energy striking the photoconductor. Such leakage is considered to be objectionably high when it appreciably affects the contrast between a picture element produced by a region of the photoconductor exposed to radiant energy, and a picture element produced by a region in the dark.

Excessive dark current is particularly objectionable in color camera's. In color cameras it is desirable that the three pickup tubes used for picking up red, green, and blue signals, have a relatively high signal to dark current ratio. Such high'signal to dark current ratio renders many differences in dark current among the three pickup tubes of tolerably small significance. However, such high signal to dark cur-rent ratio cannot be obtained by pickup tubes heretofore available.

The decay of a photoconductor constitutes a transition from a conductive to a nonconductive state after light energization has stopped. For good results, it is desirable that such decay or transition be completed within the period during which the photoconductor is subjected to a single scanning frame. If not completed during such period, the response of the photocondu-ctor to a succeeding scanning frame will be adversely affected in that the output of the device in which the photoconductor is used will have superposed thereon a portion of the image produced by the previous scanning frame. Such superpositioning of one image upon the other results in a blur in the transmitted picture.

For panchromatic response of a photoconductor containing vitreous selenium, there has been added to the selenium relatively small amounts of tellurium and arsenic. The tellurium and arsenic when used have heretofore been used fairly uniformly throughout the selenium. Such uniformity in the case of tellurium, has contributed to poor decay of the photoconductor.

Another problem concerns voltage breakdown of the photoconductive layer. In order to improve the electrical performance of the device, particularly in relation to dark current, light sensitivity, and decay characteristics, it is generally desirable to operate the signal plate of the target at a voltage which is above that tolerable without voltage breakdown of the vitreous-selenium alloy layer. The use of such high voltage has not been feasible heretofore because of the resultant voltage breakdown through the photoconductive layer.

It is an object of the invention to provide an improved photoconductor for a photoconductive device.

Another object is to provide an improved target for a photoconductive device having the several desirable characteristics of reduced dark current, and improved light sensitivity, spectral response, decay characteristics, and voltage stability.

The foregoing objects are realized in a target of a photoconductive device wherein the target comprises an insulating substrate such as the end wall of an elongated envelope, a conducting coating on the substrate for service as a signal electrode, and a layer of photoconductive material containing a vitreous alloy of selenium and tellurium, and in which the tellurium content is graded so as to provide a richer concentration of tellurium at the surface of the layer adjacent to the conducting coating than at the opposite surface. The amount of tellurium in the layer is preferably gradually reduced towards the opposite or free surface portion of the layer.

The grading of the tellurium content of the photoconductive layer in the manner indicated, accomplishes the foregoing objects in the following ways. When the conducting coating is made of a material of N-type conductivity, a junction is formed by the conducting coating and the tellurium-rich P-type surface of the photoconductive layer. This junction gives rise to constrained electron and hole mobility therethrough so that the dark current is appreciably reduced and voltage stability is increased. The gradation in tellurium content from the relatively large amount at the interface formed by the photoconductor and the conducting coating for desired blocking of dark current flow therethrough, also contributes to improved sensitivity and good red spectral response without suppressing the blue response. The relative smoothness of the tellurium gradation in the photoconductive layer avoids abrupt discontinuities that restrain electron and hole mobility, and thus contribute to a desired fast decay.

Arsenic may be added to the vitreous-selenium-telluriurn alloy in relatively small amounts for the purpose of further improving the decay characteristics of the photoconductor and for contributing to a stabilizing of the photoconductor in the vitreous state.

For further stabilizing the vitreous character of the photoconductor, stabilizing layers may be interposed between the conductive coating and the photoconductor, and

also applied to the free surface of the photoconductor.

While the junction between the photoconductive layer and the conductive coating performs an effective blocking action, it may be desirable to further improve voltage stability and further decrease the dark current beyond the relatively small amount permitted by the aforementioned blocking action. Such further improvement may be accomplished by suitably interposing a blocking layer of low-work function material in the target structure. Where no stabilizing layer is used, the layer of low-work function material may be interposed between the conducting coating and the photoconducting layer. Where a stabilizing layer is used between the conducting coating and the photoconductive layer, the blocking layer of lowwork function material may be interposed between the stabilizing layer and the photoconducting layer.

In the drawing to which reference is now made for description of an embodiment of the invention by way of example:

FIG. 1 is a fragmentary view, partly in section of a tube having a photoconductive target in the form of a vitreous alloy of selenium and tellurium and in which the tellurium content of the alloy is graded from one surface to the other of the alloy layer;

FIGS. '2, 3 and 4 are fragmentary sectional views of photoconductive targets each of which includes a layer of a vitreous alloy of selenium and tellurium, as shown in FIG. 1, and in addition FIG. 2 shows a blocking layer between one surface of the selenium-tellurium alloy layer and a signal electrode in the form of a coating or layer;

FIG. 3 shows stabilizing layers engaging both surfaces of the vitreous-tellurium alloy layer; and

FIG. 4 shows both stabilizing and blocking layers associated with the selenium-tellurium alloy layer;

FIG. 5 shows schematically apparatus that may be used in forming several of the layers shown in FIG. 4 and has on the base thereof several of the operating devices employed;

FIG. 6 is a plan view of the upper surface of the base, taken along the line 6-6 of FIG. 5 and depicts the several operating devices employed; and

FIG. 7 is a functional graph showing the difference in light absorption between a vitreous-selenium-tellurium alloy layer in which the tellurium is uniformly present, and a layer in which the tellurium content is graded.

In FIG. 1 is shown a vidicon tube 10 in which the photoconductor structure or target 12 incorporates the present invention. The tube is conventional except for the target 12. The tube 10 comprises an elongated glass envelope 14 closed at one end thereof by a transparent glass faceplate 16. The faceplate is sealed across the end referred to by means of an indium ring '18 and a clamping ring 20. The target 12 is positioned on the inner surface of the faceplate 16. Closely spaced from the target 12 is a mesh screen 22 mounted across one end of a tubular focusing electrode 24. In the other end portion (not shown) of the envelope 14 is positioned an electron gun for providing an electron beam. The electron beam is scanned across the target 12 by suitable means such as electromagnetic coils (not shown) disposed outside of the envelope 14.

Considering in more detail the structure and function of each of the several layers of the target 12 shown in FIG. 1, a layer 26 of a conducting material such as rhodium or Tic, a form of tin oxide, may have a thickness of from about 6 to about 25 Angstroms so as to be light transparent for permitting substantially unimpeded light from a scene to reach a photoconductive layer 28. This thickness also permits the layer 26 to serve as a signal electrode.

The photoconductive layer 28 may be made of a vitreous alloy of selenium, tellurium, and arsenic, and may have a thickness of from about 0.2 micron to about 5.0 microns. Although the arsenic may be omitted for satisfactory results, its presence contributes to improved decay. The layer 28 is applied in such a manner as to form a region adjacent to the layer 26 that is rich in tellurium, the relative amount of tellurium tapering off gradually towards a region remote from the layer 26. In one example, the amount of tellurium by weight in the surface region of layer 28 adjacent to the layer 26 is about 23%, while the tellurium contained in the opposite surface region of layer 28 is from O to about 10% by weight.

For good results, the vitreous alloy layer 28 may contain at its surface adjacent the layer 26, from 70 to 82% of selenium, 17 to 29% tellurium and about 1% arsenic, all by weight. However, for best results, the vitreous alloy layer 28 contained about 76% selenium, about 23% tellurium and about 1% arsenic, all by weight. When arsenic is not used, the selenium content may be increased about 1% by weight.

The combination of layers 26 and 28 shown in FIG. 1 contributes to reduced dark current through the photoconductive layer 28. The rhodium or Tic layer 26 is a material of an N-type conductivity, while the vitreous alloy layer 28 in the tellurium-rich surface portion engaging the layer 26, is a material of P-type conductivity. When layers 26 and 28 are reverse biased such as when the signal electrode layer 26 is connected to a positive voltage source and the photoconductive layer 28 is rendered negative by electrons impinging on the surface thereof remote from the layer 26, the junction between the layers 26 and 28 serves to block electron flow from the layer 28 to the layer 26 when the layer 28 is in a non-conductive state as when in the dark. However, when the photoconductive layer 28 is exposed to light, restraint to electron mobility therein is reduced and the junction between the two layers is no longer in a blocking state.

A further contribution to electron mobility in the photoconductive layer 28 when exposed to light, is effected by the gradual increase in tellurium content from the free surface of the layer to the surface in engagement with the signal electrode layer 26. The gradual character of the increase in tellurium content in layer 28 avoids the formation of junctions therein that even when the layer is exposed to the light, may introduce a restraint to electron mobility. The absence of such junctions contributes to a fast decay of the photoconductive layer 28, so that the layer is substantially free from residual charges therein that persist from one scanning frame to another.

When arsenic is used in the vitreous alloy layer 28, it is believed to be fairly uniformly distributed throughout the layer. This is due to the fact that the arsenic is initially uniformly distributed in the vitreous alloy of selenium, tellurium and arsenic used in forming the layer 28. Furthermore, when subjected to processing steps to be described, the boiling point of arsenic is sufficiently close to that of selenium so that both are evaporated substantially simultaneously. At the temperature used in the evaporating step, the arsenic combines chemically with the selenium to form arsenic triselenide. This form of arsenic coupled with its uniform distribution throughout the vitreous photoconductive layer 28, contributes to fast decay of the photoconductive layer.

While it has been shown in the foregoing that the junction between the signal electrode 26 and the tellurium-rich surface region of the vitreous photoconductive layer 28 performs a blocking function with respect to current transfer from the layer 28 to the layer 26 in the dark, and reduces dark current to a tolerable level, means for further blocking such current transfer in the dark and thereby further reducing the dark current to a very low and negligible value are provided in a target 29 shown in FIG. 2. This means may comprise a layer 30 disposed intermediate the signal electrode layer 26 and a photoconductive vitreous alloy layer 28, as shown in FIG. 2. The layer 30 may be made by evaporating a low work function material such as cerium, or cesium or pure selenium, on the signal electrode layer 26. These materials exhibit N-type conductivity and form a blocking junction with the P-type tellurium-rich surface of the photoconductive layer 28. I have found that thisjunction has a stronger blocking effect upon the current transfer from the photoconductive layer 28 to the signal electrode layer 26 in the dark, than the junction between the signal electrode 26 and the tellurium-rich surface of the photoconductor 28 as shown in FIG. 1.

It is not certain whether the cerium or cesium of blocking layer 30, after application, is in the metallic form. While the cesium or cerium initially applied to form the layer 30 is in the metallic form, it is likely that during the processing to be described, some oxidation or other chemical reaction of the metal occurs to convert at least some of this material to the oxide and/ or selenide form. This is suggested in the case of cesium by the fact that cesium is a notoriously active material and readily combines with any oxygen present in a processing ambient. In the case of cerium such oxidation is suggested by the crystal reaction of cerium with the selenium in the layer 28. When the layer 30 is formed by applying cerium, the layer 30 actually stabilizes the non-crystalline form of the selenium in the layer 28. Therefore, when cerium or cesium is applied, the layer 30 is believed to compose, at least in part, the oxide and/or selenide of the cesium or cerium employed in forming the layer. Consequently, when reference is made herein to cesium or cerium as the composition of the blocking layer 30, it is to be understood that these terms include cesium oxide or cerium oxide, cesium selenide or cerium selenide, or a combination of cesium oxide and cesium selenide or cerium oxide and cerium selenide.

The blocking layer 30 is sufiiciently thin so as to involve substantially no interference with light directed to the photoconductive layer 28 through the layer 30. The thickness of the layer 30 is from about to about 50 angstroms. The thickness of the signal electrode layer 26 and the thickness of the photoconductive vitreous alloy layer 28 may be as indicated'before herein in connection with FIG. 1.

In FIG. 3 is shown a target 31 including means for stabilizing the vitreous character of the photoconductive alloy layer 28. The stabilizing means comprises two layers 32 and 34. These layers are made of materials that not only effect the desired crystalstabilization of the two opposite surfaces of the vitreous photoconductive layer 28 of selenium, tellurium and arsenic, but the layer 34 nearest the gun (not shown) of the tube in addition,

'has no appreciable adverse effect upon the operation of the tube. The adverse effect that has to be avoided is loss of resolution in the target 12 due to excessive lateral electrical conductivity of the layer 34 of the target that is nearer the gun. Such lateral conductivity is not objectionable in the other stabilizing layer 32. However,

it is important in connection with layer 32 that this layer have sufiicient electrical conductivity in a direction normal to the surface thereof, to permit transfer therethrough of a signal charge to the signal electrode 26. This is true also in the case of stabilizing layer 34, since it is desirable that scanning electrons directed to the target 12 readily pass through the layer 34.

Materials that are suitable for use to form the underlying stabilizing layer 32 are gold, silver, copper, rhodium, palladium, germanium and germanium oxide, antimony trisulfide, gallium, gallium oxide, iridium and antimony. In my novel target I have found no evidence of chemical combination of any of these materials with .the photoconductor. Of these materials, gold is found to be best as a composition of the stabilizing layer 32, from the standpoint of mechanically stabilizing crystal activity in the adjacent surface of the vitreous alloy layer 28.

The stabilizing layer 34 on the gun side of the target 12 may be formed of a material such as germanium, germanium oxide, antimony trisulfide, or antimony trioxide. Of these materials, the best are germanium and germanium oxide. These materials possess the required low lateral conductivity coupled with adequate normal direction conductivity for desired charge transfer.

The thickness of the underlying stabilizing layer 32 is preferably about 6 angstroms, but may be from about 6 to about 30 angstroms. The thickness of the outer stabilizing layer 34 for satisfactory results should be from about 10 angstroms to about angstroms. The thickness of the photoconductive layer 28 and the signal electrode layer 26 may be as indicated in the foregoing remarks relating to FIG. 1.

It has been found that a stabilizing layer applied to only one surface of the photoconductive layer 28 greatly improves its thermal stability, that is, stability of its vitreous state when subjected to relatively high temperatures. Indeed, in some instances, it may be desirable to have one surface of the photoconductive layer 23 free of stabilizing material. However, for best thermal stability both surfaces on the photoconductive layer 28 should be so treated.

In FIG. 4 is shown a photoconductive target 35 having a signal electrode layer 26 on a glass substrate 16, a stabilizing underlayer 32, a blocking layer 30, a photoconductor 28 comprising a vitreous alloy of selenium, tellurium and arsenic, and a stabilizing overlayer 34. These layers in the combination shown in FIG. 4 may have the composition and thickness of corresponding layers described in the foregoing in relation to FIGS. 1, 2. and 3.

It will be noted that in the combination of layers shown in FIG. 4, the blocking layer 30 is interposed between the stabilizing layer 32 and the vitreous photoconductive layer 28. This disposition of blocking layer 30 is preferred for several reasons. One reason is that the junction formed between the photoconductive layer 28 and the blocking layer 30 is more effective for blocking current transfer from the photoconductive layer 28 to the signal electrode 26 than the junction between the stabilizing layer 32 and 1 the photoconductive layer 28. Another reason is that the blocking layer 32') exhibits good stabilizing characteristics and serves to stabilize the vitreous character of the photoconductive coating 28 to an appreciable degree. Due to the thinness of the blocking layer 30, i.e., from about 6 to about SOangstroms, the stabilizing layer 32 is able to contribute a further appreciable stabilizing effect through the thin blocking layer, upon the photoconductor 28. If this cumulative stabilizing effect is not required, the stabilizing layer 32 may be omitted and the blocking layer 30 may be relied on to provide the necessary blocking and stabilizing functions. However, for best results with respect to stabilization of the photoconductive layer 28, both blocking layer 30 and stabilization layer 32 may be used.

It is to be noted in this connection that the stabilizing layer 32 exhibits N-type conductivity as does the signal electrode layer 26.

In making the target 12, several layers thereof may be formed by evaporation in successive steps by a common processing equipment as shown in FIGS. 5 and 6. This equipment includes a base or platform 36 on which are mounted a plurality of evaporators (FIG. 6). Surrounding the evaporators is a tubular vapor confining structure 38 made of stainless steel, for example, and having an upper dome-shaped portion 40 provided with several openings. One of these openings is adapted to receive a glass workpiece 16 to be coated. Other openings are provided for receiving several devices for monitoring the thickness of the coatings applied to the glass workpiece 16'. A light source 41 within the enclosure 38 is positioned to direct light through the workpiece 16 and to a photocell 42 through a glass condensing lens 44, for further monitoring the thickness of the coatings applied to the workpiece 16. A bell jar 4-5 which may be made of glass, encloses chamber 38 and other processing equipment to be described, and is evacuated to a pressure of about 2X10- torr through a tubulation 46. The vapor confining structure or chamber 38 is maintained at the same pressure as that within the bell jar 45. Further features of the equipment will become apparent from the following description of the several coating operations required to form the layers shown in FIG. 4, for example. In this figure the maximum number of layers formed during a practice of the invention is shown.

Where the signal electrode layer 26 (FIG. 4) comprises rhodium, evaporator 47 may be used. When Tic is used as the signal electrode layer, the Tic may be applied to the workpiece 16 prior to its placement in the bell jar 45. The rhodium evaporator 47 comprises a tungsten filament 48 plated with rhodium. The filament 48 is mounted upon high current leads 50 which are adapted to be connected to a current source of sufiicient value to heat the filament 48 to a temperature for evaporating the plated rhodium. The rhodium evaporant not only condenses upon the glass workpiece 16 but also upon a monitoring device 51 having two spaced layers 52 and 54 of conducting material supported on an insulating substrate 56. The spaced conducting layers 52, 54 are connected to an ohmeter 58 for registering the resistance of the rhodium film deposited on the insulating substrate 56 between the two conducting layers. When this resistance reaches about 50,000 ohms, the rhodium layer on the glass workpiece 16 has a thickness of about 6 angstroms. Current supplied to the filament 48 is then stopped and the next layer 32, which may be gold, is then applied over the rhodium.

The application of the gold layer 32 is effected by evaporating gold from a gold plated tungsten filament 60 (FIG. 6) by passing suitable electric current through leads 62, 62 to which the filament is connected. A portion of the gold evaporant condenses on the workpiece 16 to form a gold layer thereon, and part becomes deposited on a quartz thickness monitor 64. This monitor may be a type commercially available from Sloan Instruments Corp., Santa Barbara, Calif, under the designation Sloan Thickness Monitor. When the monitor shows that a layer of gold having a thickness of about 6 angstroms has been deposited, heating of the filament 60 is stopped and no further deposit of gold occurs.

The blocking layer 30, when made of cesium, may be formed by flashing cesium generated by the reduction of cesium chromate in boat 64 suitably heated by electric current fed to the boat by leads 66. Since the blocking layer 30 may have a thickness within the relatively wide tolerance range of from to 50 angstroms, the thickness of the cesium layer formed over the gold layer 32 on the workpiece 16, may be determined either by an initial critical amount of cesium chromate deposited in boat 68, or by the length of time that the boat is heated. It has been found that complete flashing of approximately one milligram of cesium chromate results in the formation of a cesium blocking layer 30' having a thickness within the tolerance range indicated.

The formation over the gold layer 32 of the photoconductive vitreous alloy layer 28 comprising selenium and tellurium and wherein the amount of tellurium present is graded in concentration from one surface of the layer to the other, and also preferably including arsenic, is accompanied by the problem of providing a uniform grading in the tellurium content. It has been found that this grading may be secured by placing a critical amount of an alloy consisting by weight of from 70 to 82% selenium, 17 to 29% tellurium and 1% arsenic in an evaporating boat 68 suitably heated by electric current fed thereto through high current leads 70 and relying on the fractional evaporation that takes place because of the different boiling point temperatures of these materials. In this connection, it is to be noted that at normal atmospheric pressure the boiling point of tellurium is 1390 C. while the boiling points of selenium and arsenic are 688 C. and 615 C., respectively. It will be appreciated, however, that at the relatively low pressures in the processing apparatus described, the boiling point temperatures of the selenium, tellurium, and

arsenic will be appreciably lower, but such boiling point temperatures will be characterized by the ratio indicated for normal atmospheric pressure.

The deposit of a critical amount of the vitreous alloy in the boat 68 is important in securing a desired grading of the tellurium. If a relatively large amount of the alloy is initially deposited in the boat 68, and the boat is heated to a temperature at which the alloy evaporates, it will be evident that the initial evaporant will be rich in selenium with relatively little or no tellurium therein. The arsenic having a boiling point close to that of selenium will also be driven off substantially in the amount of its percentage relation to the selenium. At the temperature used, it is believed that the arsenic combines with selenium to form arsenic selenide.

However, if a relatively small amount of the alloy is initially deposited in the boat 68 and heated for a sufficiently long time to cause all constituents of the alloy to evaporate, the resultant evaporated layer, while initially rich in selenium, will become rich in tellurium. If the amount of alloy deposited in the boat is so small that a layer is formed of such extreme thinness that it is difficult if not impossible to segregate selenium rich portions of the layer from tellurium rich portions, the initial layer for practical purposes behaves as a tellurium rich layer.

By slowly and uniformly increasing the amount of alloy in the boat 68, a desired uniform grading in the tellurium content of the photoconductive layer 28 may be secured, that may extend from a tellurium content by weight substantially equal to 17 to 29% in one surface of the layer, to a value as low as zero in the opposite surface.

For accomplishing a critical feed of the alloy pellets to the boat 68, a dropping pellet technique in association with monitoring the temperature of the boat 68 is employed. In practicing the pellet dropping technique apparatus of the type shown in FIG. 5 is used. This apparatus comprises a turntable or rotatable platform 72 supported on a shaft 74 extending through a bearing 76 fixed to the base 36. Shaft 74 is in torque transfer relation with respect to a manually rotatable shaft 78. The turntable 72 is adapted to support a plurality of pellets 80 made of a vitreous alloy of from 70 to 82% by weight selenium, 17 to 29% by weight tellurium and 1% by weight of arsenic. In order to obtain an extremely slow rate of feed of the alloy material the pellets 80 are relatively small, each weighing about 1 milligram. Tube 82 supported through the wall of chamber 38 extends from a peripheral region of the turntable 72 to a region above and in vertical register with the boat 68. The upper end of the tube 82 is funnel shaped to facilitate a dropping of pellets thereinto from the turntable 72.

For controlling the number of pellets 80 dropped into tube 82, a fixed scraper or arm 84 is caused to engage the upper surface of turntable 72. The engagement permits the turntable to slide below the scraper. The scraper extends radially across the turntable 72. As the turntable is rotated in a counterclockwise direction, as viewed from the top of FIG. 5, the pellets will be bunched against the remote side of the scraper 84. This side is in radial register with the funneled end of tube 82 so that further bunching of the pellets causes the pellets to travel to the periphery of the turntable 72, and one or more pellets will drop into the tube 82. The number of pellets so dropped will depend upon the rate at which the turntable is rotated.

The thermal inertia of boat 68 is relatively low and contributes to a convenient monitoring of the rate of feed of the pellets 80 to the boat. The boat 68 is preferably made of tantalum having a wall thickness of from about 1 to 2 mils. Such small wall thickness renders the boat sensitive to temperature changes produced by evaporation of the charge therein. To provide a visible indication of such changes in temperature, one or more thermocouples 86 may be attached to the boat 68.

In utilizing the aforementioned apparatus in one example for forming a photoconductive layer 28 having a uniform grading of tellurium content, the boat 68 is first heated to a temperature of from about 444 to about 448 C. Initial alloy pellets 80 are dropped into the boat 68 at a rate to maintain the boat temperature between about 445 and 448 C. This relatively slow feeding rate is continued until the first one-half micron of coating thickness is applied. This thickness may be determined by the amount of absorption of light from the lamp 41 by the coated workpiece 16 as monitored by photo cell 42. The light absorption for this relatively small thickness is about 40%. The rate of pellet feed is then gradually increased until the boat temperature is reduced to 310 C. When this temperature is reached, the thickness of the coating or layer applied to the workpiece is about 1 /2 microns as determined by a light absorption of about 70%. Thereafter, the pellets 80 are fed at a rate to maintain the boat temperature at about 310 C. until the light absorption of the layer is 88% indicating a thickness of about 5 microns.

In FIG. 7 are shown two functional curves X and Y. The ordinate of the curve is in terms of light absorption calibrated logarithmically. The abscissa is calibrated in microns. No specific value of light absorption is given since the curves are merely intended to show a relationship between the photoconductors. That is to say, curve X denotes the light absorption by a layer in which tellurium content is uniform throughout the layer, while curve Y shows the light absorption by a photoconductive layer 28 in which the tellurium content is initially relatively rich and tapers off as the coating operation continues. The different slopes of curves X and Y reflect the different light absorbing properties of selenium and tellurium, the latter having an appreciably greater light absorbing characteristics than selenium.

Point a on curve Y indicates the light absorption of the layer after the first portion thereof of one-half micron thickness has been deposited. At this thickness there is substantially no difference between a uniform concentration of tellurium in the layer and a graded concentration. However, at the end of the application of a layer 1.5 microns thick of alloy material, the light absorption of the layer, when applied in accordance with this disclosure is appreciably smaller as shown by point b on curve Y. The corresponding light absorption of this thickness, if the layer were uniform in tellurium content, would be appreciably higher as shown by curve X. As the coating application continues in accordance with the present disclosure, the concentration of tellurium in the layer gradually tapers off as shown by the upper straight line portion of curve Y, until point c is reached. At this point, the coating thickness is the desired 5 microns. While the light absorption at point c on curve Y occurs at a coating thickness of 5 microns, it will be noted that substantially the same amount of light absorption takes place through a layer of uniform tellurium dispersion, when this layer is appreciably thin, i.e., about 2 microns thick. I have determined that the relative amount of tellurium in the material evaporated at the start of curve Y is about 23% by weight and that this relative amount gradually tapers off to from 0 to about 10% by weight at a point 0 of curve Y.

The amount of arsenic in the alloy used to form the layer 28 is so small that it has negligible effect upon the curves shown in FIG. 7.

When the final stabilizing layer 34 comprises germanium, it may be applied by evaporating germanium from a boat 87 supported by leads 88 adapted to lead electric current to the boat 87 for heating it to a temperature of about 1050 C. for evaporating the germanium therein (FIG. 6). The germanium evaporant condenses in equal thickness amounts onto the workpiece 16 and on the 1G quartz thickness monitor 64. When the monitor indicates that .a germanium coating having a thickness of from about 10 to about 100 angstroms has been formed by the germanium evaporant, further heating of the boat is stopped and further coating application ceases.

After the several coatings have been applied in the manner indicated in the foregoing, the coated workpiece 16 may be incorporated in the tube shown in FIG. 1 by being sealed thereto by indium ring 18 to form a faceplate. This ring requires a relatively low sealing temperature so as not to harm the coatings formed on the former workpiece and now faceplate 16.

It will be apparent that when the coated target structure shown in FIG. 1 is desired and the signal electrode 26 comprises Tic formed outside of the bell jar, only one step is required with the aid of the apparatus shown in FIGS. 5 and 6. This one step comprises forming the photoconductor layer 28 with a tellurium content that is graded from one surface to the other of the layer. However, the apparatus described may be used for applying additional coatings as shown in FIGS. 2 to 4.

What is claimed is:

1. A photoconductive device having a target compris- (a) a conducting signal electrode made of a material of N-type conductivity, and

(b) a photoconductive layer over said signal electrode,

(1) said photoconductive layer comprising a vitreous alloy of selenium and tellurium, which has a higher percentage of tellurium at the surface portion thereof adjacent to said signal electrode than at the opposite surface portion thereof whereby a blocking junction is formed between said signal electrode and said photoconductive layer, for reducing the dark current of said device.

2. A photoconductive device according to claim 1 and wherein:

(a) said photoconductive layer has a thickness from about 2 to about 5 microns, and

(b) said surface portion of higher percentage of tellurium includes by weight about 23% tellurium and the opposite surface portion thereof includes from 0 to about 10% by weight of tellurium.

3. A photoconductive device according to claim 1 and wherein the amount of tellurium in said photoconductive layer is substantially uniformly reduced from said firstnamed surface portion to said opposite surface portion, whereby said photoconductive layer is free of any junction therein of materials of different conductivity type.

4. A photoconductive device according to claim 1 and wherein said photoconductive layer includes about 1% by weight of arsenic.

5. A photoconductive device having a target comprising:

(a) a conducting signal electrode made of a material of N-type conductivity,

(b) a layer of low work function material over said signal electrode, and

(c) a vitreous photoconductive layer over said layer of low work function material,

( 1) said photoconductive layer comprising selenium having a relatively large amount of tellurium in a first surface region thereof adjacent to said layer of low work function material said photoconductive layer exhibiting P-type conductivity, and a relatively small amount of tellurium in a second surface region thereof remote from said layer of low work function material.

6. A photoconductive device according to claim 5 and wherein said layer of low work function material comprises cesium, cerium or pure selenium.

7. A photoconductive device according to claim 5 and wherein the amount of tellurium present in said photoconductive layer is gradually richer throughout the thickness of said photoconductor from said second surface region to said first surface region thereof.

8. A photoconductive device having a target compris- (a) a conducting signal electrode made of a material of N-type conductivity,

(b) a layer of low work function material over said signal electrode,

(0) a vitreous photoconductive layer over said low work function material layer comprising selenium having therein an amount of tellurium that is substantially uniformly decreased through the thickness of said photoconductive layer in the direction away from the surface thereof adjacent to said layer of low work function material said photoconductive layer exhibiting P-type conductivity, and

(d) a stabilizing layer between said signal electrode and said low work function material layer, for stabilizing the vitreous character of said photoconductive layer.

12 9. A photoconductive device according to claim 8 and wherein said stabilizing layer comprises gold, silver, copper, rhodium, palladium, germanium and germanium oxide, antimony trisulfide, gallium, gallium oxide, iridium, or antimony.

10. A photoconductive device according to claim 8 and wherein a second stabilizing layer is disposed over said photoconductive layer and comprises one of germanium, germanium oxide, antimony trisulfide or antimony trioxide.

References Cited UNITED STATES PATENTS 3,310,700 3/1967 Dresner et a1 .31394 X DAVID I GALVIN, Primary Examiner.

R. SEGAL, Assistant Examiner. 

1. A PHOTOCONDUCTIVE DEVICE HAVING A TARGET COMPRISING: (A) A CONDUCTING SIGNAL ELECTRODE MADE OF A MATERIAL OF N-TYPE CONDUCTIVITY, AND (B) A PHOTOCONDUCTIVE LAYER OVER SAID SIGNAL ELECTRODE, (1) SAID PHOTOCONDUCTIVE LAYER COMPRISING A VITREOUS ALLOY OF SELENIUM AND TELLURIUM, WHICH HAS A HIGHER PERCENTAGE OF TELLURIUM AT THE SURFACE PORTION THEREOF ADJACENT TO SAID SIGNAL ELECTRODE THAN AT THE OPPOSITE SURFACE PORTION THEREOF WHEREBY A BLOCKING JUNCTION IS FORMED BETWEEN SAID SIGNAL ELECTRODE AND SAID PHOTOCONDUCTIVE LAYER, FOR REDUCING THE DARK CURRENT OF SAID DEVICE. 